Frictionless and low-friction bearings are useful in a variety of mechanical instruments, including actuators for the movement of magnetic recording heads in computer disk file systems. One type of bearing which is known to be suitable for this purpose is the externally-pressurized air bearing, i.e. one in which two surfaces are prevented from touching by the flow of air from an external air pump. However, the poor reliability and high cost of such pumps has prevented this type of bearing from finding acceptance in disk files.
A type of bearing which might initially appear to be suitable is the squeeze bearing taught by E. O. Salbu as discussed further below. In this bearing, two closely conforming surfaces are oscillated next to each other to generate a positive load supporting force. However, these bearings, first described in a printed publication in 1964 have a number of drawbacks which have made them unsuitable for a practical application in most technologies:
1. The conforming surfaces are rigid and heavy
2. The transducer which oscillates the surfaces of the bearing must be correspondingly massive.
3. The power dissipation is high.
4. The ratio of bearing weight to load-carrying capacity is too large.
5. Because of the relatively large oscillating mass of the bearing itself, it has been impossible to make the frequency of oscillation higher than the audible range. In fact the previous squeeze bearings designed in accordance with the teachings of Salbu have produced acoustic vibrations which are intolerably loud.
6. The oscillatory forces cause excessive vibration of the object supported by the bearing which is frequently an intolerable characteristic of a bearing, especially where precise location of the object is required, as contrasted to oscillation of location of the object.
U.S. Pat. No. 3,626,510 of Kauzlarich et al for "Hydraulic Bearing System" describes a gyroscope with a hydraulic thrust bearing with a liquid between bearing surfaces, one of which is oscillated normal to the bearing surfaces to generate a positive load supporting force on the bearing. The patent cites the work of E. O. Salbu for "Compressible Squeeze Films and Squeeze Bearings" Trans. ASME Journal of Basic Engineering, June 1964 characterized as teaching that the "Reynolds Equation is used to show that the relative lateral motion between the bearing surfaces produces a nonsymmetrical force per cycle with the greater force occurring on the closing part of the cycle. Such phenomenon is referred to as `pumping action` provided by the high frequency oscillations and is attributable to compression occurring in a compressible gas film." Such bearings are described as squeeze bearings with a pair of parallel surfaces which derive support by relative axial oscillation of the surfaces at a high frequency. It states later that at high frequency, the gas cannot flow out of the bearing space fast enough to avoid compression. It is also referred to as pumping action attributable to compression occurring in the compressible gas film. Various references are cited there. The Kauzlarich et al patent is a hydraulic derivation of the gas squeeze bearing, with an anticavitation means in the form of a pressurizing means, combined with a partially hemispherical air squeeze bearing at the opposite (upper) end of the shaft. The hydraulic bearing (less curved) is at the bottom. Both are thrust bearings. The lower bearing has a pivot which prevents the lower end of the shaft from coming to rest on the seat.
U.S. Pat. No. 3,339,421 of Warnock for a "Dynamic Gas Film Supported Inertial Instrument" describes a cylindrical PZT piezoelectric cylinder providing gaseous squeeze bearing about the periphery of the cylinder and on its axial ends in a gyroscopic mounting.
U.S. Pat. No. 3,433,538 of Blanding et al "Dynamic Gas Film Bearing Structure" describes an improved air bearing with a resilient ring-like element in the bearing for support of the piezoelectric transducer element along one of the strain axes of the element.
U.S. Pat. Nos. 3,471,205 and 3,359,045 of Farron et al and Hsu, respectively, are both entitled "Squeeze Film Bearings". A cylindrical piezoelectric or magnetostrictive strain producing member vibrates in both axial and radial direction at an ultrasonic frequency. The device provides a simultaneous thrust and journal bearing.
U.S. Pat. No. 4,099,211 of Hathaway "Positionable Transducing Mounting Structure and Driving System Therefor" describes a "bimorph" suspension for a magnetic transducing head. The bimorph is a piezoelectric bender leaf element flexible in lateral directions having axes of polarization extending in the lateral directions. The bimorph is two flat piezoceramic plates bonded to an intervening substrate which bend up or down along the length of a long thin leaf element. On the end of the leaf element is a magnetic recording head for reading an omega video tape drive. The leaf can be doubly articulated with bimorph elements end-to-end bending the leaf in opposite directions in each half. It is an actuator, not a bearing, of course. It does, however, use a bending laminated piezoelectric stack in connection with magnetic recording heads.
U.S. Pat. No. 4,106,065 of Ravizza is for drive circuits for the bimorph actuator of the head of the preceding patent.
U.S. Pat. No. 4,188,645 of Ragle et al for "Piezoelectric Servo for Disk Drive" describes a fine positioning mechanism for a disk read head using several pairs of opposed flexing piezoelectric bending elements to move a read head arm at right angles to the length of the bender elements. The bender elements in each pair are not laminated but are mounted in parallel. With two spaced elements above and below the arm, all bending in the same direction all four elements drive the head in the same direction.
U.S. Pat. No. 3,351,393 of Emmerich for "Piezoelectric Oscillating Bearing" describes a jewel bearing supported pair of disks of oppositely internally polarized piezoelectric elements operative so that (Col. 2 line 60) ". . . as one disk expands the other contracts so that the motion of the bearing 24 supported on the disks is relatively large as compared to that which would be produced by a single oscillating piezolectric support." It mentions gyroscopes and the need for vibrating bearings which reduce friction but it makes no mention of squeeze bearings and seems to be limited to the concept of vibratory mechanical motion caused by impact of the solid materials upon the support for the bearings. The frequency of the oscillators 46, 48 and 58 is not specified.
U.S. Pat. No. 1,860,529 of Cady shows that stacked, laminated, piezoelectric resonators are very old in the art. It does not describe vibrating disks.
U.S. Pat. No. 3,114,848 of Kritz describes a "High Efficiency Sonic Generator" with a piezoelectric sandwich of quartz disks mounted at a nodal circle of the flexing disk. The use is not relevant since it is simply desired to vibrate air, and the device is not employed in bearings or for supporting a structure of any kind.
U.S. Pat. No. 3,304,132 of Broeze et al describes a laminated-disk bearing with annular rings of piezoelectric elements on opposite surfaces of the disk at each end of a gyroscope. The piezoelectric elements are distorted to oscillate the bearings in the direction of the axes of the shafts. The bearings are journal bearings. Broeze U.S. Pat. No. 3,239,283 is a variation on the idea which is less relevant.
Hall U.S. Pat. No. 2,993,739 for a "Magnetostrictive Bearing Assembly" covers a thrust bearing arrangement with opposing magnetostrictive elements vibrating a set of thrust bearings of a sensitive instrument. The magnetostrictive elements are rods which produce oscillations between the two bearings carrying the movement of the instrument.
It appears that the first feature of the disk bearing with cupping and reverse cupping is novel, in view of the above references. The bearing art relative to piezoelectric vibration which involves mechanical contact during operation is fairly crowded. The art of squeeze bearings is not so developed. Cupping bearings have not been used for squeeze bearings. The prior art is replete with bearings involving multiple combinations of piezoelectric vibration. The concept of applying the Kritz or Broeze laminated bearing to the teachings of Salbu has not been found in the prior art, although over 13 years has elapsed since those teachings were published. No prior art has been found on the distorted sleeve bearing. The use of nodal mounting in a vibrating squeeze bearing is also novel.
I examined the possibility of using Salbu type squeeze film bearings for a totally new application: to support the voice coil and head suspension spring of a disk file actuator. The use of a non-contact, internally damped bearing for the support of the voice coil would solve the striction and friction problems of future disk file actuators.
The type of squeeze film bearing discussed by Salbu requires one of the bearing surfaces to vibrate against a thin entrapped air region to produce a squeeze film of superambient pressure. From considerations of load, stiffness and machining tolerances, a minimum vibration amplitude of 2.5 .mu.m is necessary.
Two known methods of producing such a vibration are magnetic and piezoelectric. However, a magnetic actuator produces stray fields and is only capable of "unidirectional motion"--i.e., a thrust bearing. It is not easily used for a journal bearing. Unfortunately, the simple expansion and contraction of a piezoelectric disk is a very small effect. To produce the required amplitude of vibration would require a very thick and undesirably massive transducer (see FIG. 1), even in resonance.